1. Field of the Invention
The present invention relates to a communication device for carrying out the process of releasing a data link when a call is released, and a method and a program for controlling the communication device.
2. Related Background Art
In the D channel of an integrated services digital network (ISDN), a data link is set in its layer 2, and a network layer is set in its layer 3.
In the control of the layers 3 and 2, when calling or communications are finished, and a release-complete (REL-COMP) message is sent or received to release a call, the layer 3 makes a data link releasing request to the layer 2 by notifying Data Link (DL)-Release-Request, if no other currently used calls are present at a point of this time. Upon having received the DL-Release-Request, the layer 2 sends a disconnect (DISC) command as a data link cutting-off instruction to the network. After the reception of an unnumbered acknowledge (UA) response as a response to the command, the layer 2 outputs DL-Release-Confirmation to the layer 3, thereby notifying the releasing of the data link.
Upon having received the DISC command, an opposite communication side notifies DL-Release-Indication from the layer 2 to the layer 3 to release the data link, and sends the UA response to the network.
In the conventional device, a time from the releasing of a call to the request of data link releasing made by the layer 3 to the layer 2 takes a value intrinsic to the device.
However, depending on a network (switching device) to be used, if the DISK command of the layer 2 was sent without any proper time interval taken after the transmission or reception of the REL-COMP message of the layer 3, the data link releasing process was not finished normally, bringing about the resetting of a data link by the network.
Furthermore, since a permissible value for such a time interval varied from network to network (switching device), it was impossible to normally finish call releasing depending on a device installing place (switching device, to which the device was connected).